Preclinical Studies in Pancreatic Cancer and Melanoma Demonstrated ADI-PEG 20 Works Synergistically With Inhibitors of Autophagy

Data Presented at the 102nd Annual Meeting of the American
Association for Cancer Research

SAN DIEGO, April 11, 2011 /PRNewswire/ -- Polaris Group
(Polaris) announced today that data from preclinical studies of
ADI-PEG 20, the Company's pegylated arginine deiminase therapeutic,
is synergistic with hydroxychloroquine (ChQ) in a human pancreatic
cancer cell line and with tumor necrosis factor-related
apoptosis-inducing ligand (TRAIL) in human melanoma cell lines.
Both ChQ and TRAIL are known inhibitors of autophagy. These results
support the commencement of Phase 1/2 clinical studies in these
cancer types to investigate the combination of ADI-PEG 20 with
autophagy inhibitors.

"We are excited to learn that ADI-PEG 20 can potentially be used
in combination with yet another class of chemotherapeutic agents,"
commented John Bomalaski, M.D.,
executive vice president, medical affairs of Polaris. "ADI-PEG 20
has already demonstrated apparent efficacy and remarkable safety in
clinical trials as a single agent. We are now making plans to
initiate clinical studies to test the efficacy and safety of
ADI-PEG 20 when used in combination with antitumor agents and other
compounds."

A majority of human pancreatic adenocarcinomas are deficient in
argininosuccinate synthase (ASS) and are therefore sensitive to
arginine deprivation by ADI-PEG 20. ASS-deficient cell lines
undergo cell death when deprived of arginine, but also undergo
autophagy. It is hypothesized that autophagy is protective in the
setting of arginine deprivation and that inhibition of autophagy by
hydroxychloroquine (ChQ) would increase cell death. The human
pancreatic cancer cell line MIA-PaCa2 was treated in vitro
and in vivo with ADI-PEG 20 alone or in the presence of ChQ.
In vitro treatment with ADI-PEG 20 induced caspase-dependent
cell death as well as autophagy. ChQ at low doses inhibited
autophagy induced by ADI-PEG 20, but increased cell death. In
xenografts, addition of ChQ to a sub-therapeutic dose of ADI-PEG 20
increased tumor suppression, decreased autophagy and increased cell
death compared to either treatment alone. Arginine deprivation by
ADI-PEG 20 induces autophagy and cell death in cell lines deficient
in ASS1. Autophagy appears to play a protective role, and
its inactivation by ChQ enhances tumor suppression. Combining ChQ
with ADI-PEG 20 in pancreatic cancer may increase the cell death
induced by ADI-PEG 20 by blocking ADI-PEG 20-induced autophagy.

Melanomas that do not express the gene that codes for ASS
(ASS1) cannot make arginine from citrulline; thus arginine
depletion caused by ADI-PEG 20 results in growth inhibition and
eventually leads to cell death. However, autophagy does take place
during arginine deprivation in melanoma which prevents death in
some melanoma cells. During this period, certain melanoma cells can
turn on the ASS1 gene and hence become resistant to ADI-PEG
20. It has previously been shown that combining tumor necrosis
factor-related apoptosis-inducing ligand (TRAIL) with ADI-PEG 20
can accelerate the cell death (apoptosis) process and hence greatly
enhance cytotoxicity. In this study, further data from combination
treatment confirm that the enhancement effect of the combination
needs the participation of both the extrinsic and intrinsic
pathways of apoptosis. This finding suggests that the combination
treatment turns off the autophagic process during arginine
deprivation, possibly through caspase activation, and allows the
apoptotic process to take place. Overall, these findings indicate
that autophagy is a pro-survival mechanism in melanoma cells under
nutritional stress, and interfering with this pathway may achieve
greater efficacy with ADI-PEG 20 treatment.

About ADI-PEG 20

ADI-PEG 20 is a biologic being developed by Polaris to treat
cancers carrying a major metabolic defect that renders them, unlike
normal cells, unable to make arginine internally. Because arginine
is one of the 20 amino acids that are essential for protein
synthesis and survival of cells, these cancer cells become
dependent upon the external supply of arginine to survive and grow.
ADI-PEG 20 works by systemically depleting the external supply of
arginine which causes these arginine-dependent cancer cells to die
while leaving the normal cells unharmed.

Multiple cancers have been reported to have a high degree of
arginine-dependency. Phase 2 clinical trials have yielded positive
results in patients with hepatocellular carcinoma or metastatic
melanoma, and Phase 2 trials for small cell lung cancer and
mesothelioma are currently ongoing. Polaris also plans to initiate
clinical studies in prostate cancer, pancreatic cancer, leukemia,
lymphoma and sarcoma this year.

About Polaris Group

Polaris Group is a privately held multinational
biopharmaceutical company that specializes in the research and
development of protein drugs to treat cancer and other debilitating
diseases. The company's lead therapeutic, ADI-PEG 20, is advancing
into a pivotal Phase 3 trial for hepatocellular carcinoma. Polaris
is also investigating ADI-PEG 20 as a treatment for other
arginine-dependent cancers, such as melanoma, prostate cancer,
leukemia, lymphoma, sarcoma and pancreatic cancer. In addition to
the ADI-PEG 20 project, Polaris is researching and developing other
biotherapeutic agents and has a small molecule drug program that
utilizes a rational structure-based approach to design novel
compounds that inhibit the biological function of cancer-related
protein targets.